1. Technical Field
The present subject matter relates to lighting. More specifically it relates to controlling the color temperature of a light that uses light emitting diodes (LEDs).
2. Description of Related Art
Current multi-colored light sources may utilize multiple LEDs. In the simplest case, a dual color LED consists of two LED die, each of which emits a different color of light. A more variable multi-colored light source utilizing LEDs may be built using a plurality of LEDs of a variety of colors, commonly some number each of red, green and blue LEDs. A controller may be included that can individually control the intensity of each color of LED or even control the intensity of each individual LED. This allows the controller to generate a wide variety of colors.
A conventional LED die generally emits light in a narrow band of wavelengths. If that wavelength is in the visible range, this gives the LED a distinct color to a human eye. To generate a broader spectrum of light, such as needed to generate a light perceived as “white” by the human eye, a technique may be used where a narrow range of wavelengths generated by a single LED die irradiates and excites a phosphor material to produce visible light, often referred to as a phosphor LED (or PLED). The phosphor may include a mixture or combination of distinct phosphor materials, and the light emitted by the phosphor can include a variety of narrow emission lines distributed over the visible wavelength range such that the emitted light appears substantially white to the human eye.
One example of a phosphor LED is a blue LED illuminating a phosphor that converts blue to both red and green wavelengths. A portion of the blue excitation light is not absorbed by the phosphor, and the residual blue excitation light is combined with the red and green light emitted by the phosphor. Another example of a phosphor LED is an ultraviolet (UV) LED illuminating a phosphor that absorbs and converts UV light to red, green, and blue light.
Different combinations of distinct phosphor materials may give off subtle variations of spectra to emit “white” light at different color temperatures. The correlated color temperature (often simply referred to as color temperature herein) of a light source is the temperature of an ideal black-body radiator that radiates light that is perceived by the human eye to be of a comparable hue to that light source. The temperature is conventionally stated in units of absolute temperature, kelvin (K). Higher color temperatures (5000K or more) are called cool colors (blueish white); lower color temperatures (2000-4000K) are called warm colors (yellowish white through reddish white). While light with a wide range of color temperatures may still be called “white”, in reality a white light at 6000K (similar to typical daylight) is actually a different color than a white light at 3000K (similar to an incandescent bulb) or a white light at 9000K (similar to a computer CRT screen). Thus an application needing to adjust the color temperature of a light source may actually require a multi-color light source.
Many applications today would like to be able to adjust the color of the light source or the color temperature of a white light source for its artistic or psychological effects. For non-LED based lighting sources, this has often been done with filters or gels placed over conventional lights. With a variety of filters, a wide variety of different colors (including different color temperatures) can be realized from a conventional lamp. Multi-colored LED light sources utilizing several different colors of LEDs have become popular due to the wide range and fine control that can be achieved using the controller. But if a limited range of finely controlled colors is required, a full set of LEDs with their associated controller may be too expensive and bulky for many applications and even then, the limited spectral content available from LEDs may not provide the ability to create subtle differences in perceived color such as slight variations in color temperature.